Method to deposit functionally graded dielectric films via chemical vapor deposition using viscous precursors

ABSTRACT

A method of forming a graded dielectric layer on an underlying layer including flowing a mixture of a silicon-carbon containing gas, an oxygen containing gas and a carrier gas through a showerhead comprising a blocking plate and a faceplate to form an oxide rich portion of the graded dielectric layer, where the silicon-carbon containing gas has an initial flow rate, flowing the silicon-carbon containing gas at a first intermediate flow rate for about 0.5 seconds or longer, where the first intermediate flow rate is higher than the initial flow rate, and flowing the silicon-carbon containing gas at a fastest flow rate higher than the first intermediate flow rate to form a carbon rich portion of the graded dielectric layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/931,609, entitled “METHOD TO DEPOSIT FUNCTIONALLY GRADED DIELECTRICFILMS VIA CHEMICAL VAPOR DEPOSITION USING VISCOUS PRECURSORS,” filedSep. 1, 2004, the entire disclosure of which is incorporated herein byreference for all purposes.

BACKGROUND OF THE INVENTION

Conventional integrated circuit (“IC”) device fabrication includesetching patterns of gaps into a metal layer such as aluminum. The gapsmay then be filled with dielectric materials such as silicon dioxide.More recently, IC device fabricators are switching from aluminum tocopper and other more conductive metals to take advantage of the lowerresistance of these metals to electric current. In the case of copper,the metal's higher resistance to etching than aluminum is encouraging aswitch to damascene processes where dielectric layers are deposited toform an integrated stack that are etched to create gaps for a subsequentmetal gap-fill.

The dielectric layers that separate the layers of metal in a damascenestructure are sometimes referred to as intermetal dielectric (IMD)layers. The capacitance (C) of the IMD material and the resistance (R)of the metal layers are significant components of the RC constant of theIC circuit. As the RC constant decreases, the circuit speed increases,and IMD layers having lower capacitance (i.e., lower dielectricconstants “κ”) complement the lower resistance of metals like copper.

IMD layers typically include a barrier layer to prevent the diffusion ofthe metal into the adjacent dielectric layers. One material used for thebarrier layer is silicon nitride (Si_(x)N_(y)), which is also commonlyused as an etch stop material for the formation of the damascenestructures. Unfortunately, silicon nitride has a relatively highdielectric constant (κ=7.0 to 7.5 for Si₃N₄ compared to κ=4.0 to 4.2 forSiO₂), which increases the overall κ value of the dielectric layer.

More recently, barrier layers have been developed from materials withlower dielectric constants. Silicon-carbon based barrier layers (e.g.,silicon oxycarbide (SiOCH) barrier layers) have been developed that havelower dielectric constants than silicon nitride. One such layer, forexample, is the BLOK™ (Barrier Low K) developed by Applied Materials,Inc. of Santa Clara, Calif. These low-κ barrier layers may be depositedby, for example, plasma enhanced chemical vapor deposition usingtrimethylsilane (TMS).

While silicon oxycarbide and other silicon-carbon based low-κ barrierlayers have improved dielectric constants, they often have pooradherence to other low-κ silicon-carbon materials that make up the bulkdielectric portion of the IMD layer. Oxide films such as silicon dioxide(SiO₂) adhere much better to the silicon-carbon based low-κ barrierlayers, but also have higher κ values that raise the overall dielectricconstant of the IMD layer. Thus, there is a need for methods of forminglow-κ IMD layers that have good adhesion between the barrier layer andthe bulk dielectric portion of the layer.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention include a method of forming agraded dielectric layer on an underlying layer. The method includes thestep of flowing a mixture of a silicon-carbon containing gas, an oxygencontaining gas and a carrier gas through a showerhead comprising ablocking plate and a faceplate to form an oxide rich portion of thegraded dielectric layer, where the silicon-carbon containing gas has aninitial flow rate. The method also includes the step of flowing thesilicon-carbon containing gas at a first intermediate flow rate forabout 0.5 seconds or longer, where the first intermediate flow rate ishigher than the initial flow rate. The method may also include the stepof flowing the silicon-carbon containing gas at a fastest flow ratehigher than the first intermediate flow rate to form a carbon richportion of the graded dielectric layer.

Other embodiments of the invention include a method of forming a gradeddielectric layer on an underlying layer, where the method includes thestep of flowing a mixture of a silicon-carbon containing gas, an oxygencontaining gas and a carrier gas through a showerhead comprising ablocking plate and a faceplate to form an oxide rich portion of thegraded dielectric layer, wherein the silicon-carbon containing gas hasan initial flow rate. The method also includes the step of increasingthe silicon-carbon containing gas to a fastest flow rate to form acarbon rich portion of the graded dielectric layer, where the carriergas has a carrier gas flow rate that stays constant until after thesilicon-carbon containing gas reaches the fastest flow rate.

Additional embodiments of the invention include a system for forming agraded dielectric layer on an underlying layer. The system includes ashowerhead that includes a blocking plate and a face plate, wherein theshowerhead is coupled to a gas supply inlet through which a process gascomprising a silicon-carbon containing gas, an oxygen containing gas anda carrier gas is introduced into the showerhead. The system alsoincludes a liquid flow meter to control a flow rate of thesilicon-carbon containing gas to the showerhead, where the liquid flowmeter is programmed to flow the silicon-carbon containing gas at aninitial flow rate during the formation of an oxygen rich portion of thegraded dielectric layer, then increasing the silicon-carbon containinggas flow rate from the initial flow rate to an intermediate flow rate,and maintaining the intermediate flow rate for about 0.5 seconds orlonger, and then further increasing the silicon-carbon containing gasflow rate from the intermediate flow rate to a fastest flow rate to forma carbon rich portion of the graded dielectric layer.

Additional features are set forth in part in the description thatfollows, and in part will become apparent to those skilled in the artupon examination of the following specification or may be learned by thepractice of the invention. The features and advantages of the inventionmay be realized and attained by means of the instrumentalities,combinations, and methods particularly pointed out in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a barrier layer and functionally graded dielectriclayer formed on a metal filled trench according to embodiments of themethods of the present invention;

FIG. 2 shows a simplified cross-sectional view of an integrated circuitdevice that includes functionally graded dielectric layer formedaccording to embodiments of the methods of the present invention;

FIG. 3 shows a flowchart of steps for forming a graded dielectric layeraccording to embodiments of the methods of the present invention;

FIG. 4 shows a flowchart of steps for forming a graded dielectric layeraccording to additional embodiments of the methods of the presentinvention;

FIG. 5 plots the flow rate of a silicon-carbon containing gas as afunction of time for a conventional dielectric layer deposition and adeposition according to an embodiment of the methods of the presentinvention;

FIG. 6 shows a sectional view of a showerhead that may be used withembodiments of the present invention; and

FIG. 7 shows a simplified cross-sectional diagram of an exemplary plasmaCVD deposition chamber used with embodiments of the systems and methodsof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention include methods of forming a low-κ,functionally graded film layer that includes an oxide rich portion forbetter adherence to an underlying layer (e.g., a silicon oxycarbidebarrier layer) and a carbon-rich portion (e.g., the bulk dielectric)having a lower κ value. The method includes flowing a mixture ofsilicon-carbon containing gas and oxygen containing gas at an initialflow rate favoring a high oxygen to silicon flow rate ratio to deposit athin (e.g., about 10 Å to about 80 Å thick) oxide rich portion of thefunctionally graded dielectric layer. Afterwards, flow rate for thesilicon-carbon containing gas may be increased (and oxygen containinggas may be decreased) to boost the silicon-carbon concentration in thebulk dielectric portion of the functionally graded dielectric layer.Because the oxide-rich portion is relatively small, it has a smalleffect on the overall κ value of the layer.

The silicon-carbon gases used with the present invention are oftenorgano-silicon compounds such as octamethylcyclotetrasiloxane (“OMCTS”)[((CH₃)₂SiO)₄]. Compounds like OMCTS are viscous liquids at roomtemperature, which when heated (e.g., about 350° C.) produce vapors thatmay be carried by a carrier gas to a deposition chamber where thefunctionally graded dielectric is formed.

The viscous liquid phase characteristics of these organo-siliconcompounds at room temperature create challenges for delivering a uniformdistribution of their vapors through a gas showerhead and into thedeposition chamber. The inertia effects of the vapors favor their beingmore concentrated near the center of the showerhead and lessconcentrated at towards edges. As a consequence, a blocker plate (alsoknown as a baffle) may be inserted between the showerhead gas supplyinlet and faceplate to distribute more evenly the silicon-carbon vaporsthat pass through the faceplate into the deposition chamber.

The blocker plate, however, can also introduce a drop in the pressure ofthe gas mixture across the plate, which can result in condensation ofthe silicon-carbon around passages through the blocker plate and faceplate. The condensate in these passages, as well as the formation oforgano-silicon aerosol particles in the reaction chamber, can lead toincreased numbers of infilm adders (e.g., about 10,000 or more adders)falling on the substrate. The problem is exacerbated by the fact thatthe adders are not uniformly distributed on the substrate, but insteadtend to concentrate in the area below the center of the showerheadfaceplate.

Embodiments of the present invention reduce condensation of thesilicon-carbon vapor and the infilm adders through a number oftechniques, including modulating the change in the flow rate of thesilicon-carbon containing gas, and maintaining a high carrier gas flowrate until after an increase in the flow rate of the silicon-carboncontaining gas has occurred. These techniques and others, used alone orin combination, can reduce the number of infilm adders from the tens ofthousands to the order of about 10 or less. Before describingembodiments of the techniques in more detail, a description of somesimplified IC structures that include the functionally graded dielectriclayers is presented.

Exemplary Integrated Circuit Structures

FIG. 1A shows an example of a layers upon which a barrier layer and afunctionally graded dielectric layer formed according to embodiments ofthe present invention. The layers include a dielectric layer 104containing trench 106 filled with a conductive material (e.g., copper).The dielectric layer 104 and trench 106 may be formed on an underlyingsupport layer 102. Support layer 102 may be a silicon substrate, a metalfilm layer (e.g. a copper or aluminum layer), a salicide layer formed ontop of a source, drain or gate electrode, or a dielectric layer (e.g., aIMD layer), among other kinds of layers.

Referring to FIG. 1B, a barrier layer 108 and functionally gradeddielectric layer 110 is shown formed on the underlying layer of FIG. 1A.Barrier layer 108 may be formed from silicon-carbon based materials(e.g., silicon oxycarbide) that have lower κ values than conventionaloxide and nitride based barrier layers (e.g., silicon nitride). The κvalue of barrier layer 108 formed from silicon-carbon based materialsdepends on the ratio of carbon to silicon atoms in the layer.

A barrier layer 108 having an atomic ratio of carbon to silicon (C:Siratio) from about 55:45 to about 65:35 normally provides a layer withlow κ characteristics. If the C:Si ratio in the layer is too low, thedielectric constant may be too large (e.g., κ=7.0 for a C:Si ratio ofabout 50:50). In addition, the leakage current of the layer tends toincrease as the C:Si ratio in the layer decreases. The C:Si atomic ratioin the layer depends in turn on the C:Si atomic ratio of the gaseousmixture used to form the layer.

Generally, the C:Si ratio in the gaseous mixture is greater than 1:1(e.g., about 3:1 to about 8:1). For a given C:Si ratio in the gaseousmixture, the higher the substrate temperature the lower the C:Si ratioin the deposited layer. Relatively high C:Si ratios in the gaseousmixture may be desirable when depositing the barrier layer 108 at highsubstrate temperatures. Details of the deposition of exemplary barrierlayers upon which a functionally graded dielectric layer may be formedaccording to methods of the invention are described in commonly assignedU.S. Pat. No. 6,713,390, filed on Jul. 12, 2002, entitled “BARRIER LAYERDEPOSITION USING HDP-CVD,” the entire contents of which are hereinincorporated by this reference.

Functionally graded dielectric (“FGD”) layer 110 may be formed on theunderlying barrier layer 108 according to embodiments of the methods ofthe invention described below. The FGD layer 110 may include anoxide-rich portion that is in contact with the underlying barrier layer108, and a carbon-rich portion (i.e., the bulk dielectric portion) thatis further away from barrier layer 108. The oxide-rich portion of thelayer may comprise silicon (Si), oxygen (O) and carbon (C) where theratio of silicon to oxygen is lower than average to reflect the higherthan average concentration of oxygen present in the layer. Theoxide-rich portion of the FGD layer 110 may be in a range from about 10Å to about 100 Å thick (e.g., about 70 Å to 80 Å thick).

The carbon-rich, bulk dielectric portion of FGD layer 110 may have aC:Si ratio substantially similar to barrier layer 108 (e.g., about 55:45to about 65:35). The bulk dielectric portion may be from about 5000 Å toabout 10,000 Å thick, and provide FGD layer with an overall κ value ofabout 3.5 or less (e.g., from about 3.3 to about 3.0).

FIG. 2 illustrates a simplified cross-sectional view of an integratedcircuit 200 that includes functionally graded inter-metal dielectric(FGIMD) layers 240, 242, 244 formed according to embodiments of thepresent invention. Integrated circuit 200 includes NMOS and PMOStransistors 203 and 206, which are separated and electrically isolatedfrom each other by a field oxide region 220 formed by local oxidation ofsilicon (LOCOS), or other technique. Alternatively, transistors 203 and206 may be separated and electrically isolated from each other by ashallow trench isolation (STI) technique (not shown) when transistors203 and 206 are both NMOS or both PMOS. Each transistor 203 and 206comprises a source region 212, a drain region 215 and a gate region 218.

A premetal dielectric (PMD) layer 221 separates transistors 203 and 206from metal layer 240 with connections between metal layer 240 and thetransistors made by contacts 224. Metal layer 240 is one of four metallayers, 240, 242, 244 and 246, included in integrated circuit 200. Eachmetal layer 240, 242, 244, and 246 is separated from adjacent metallayers by respective barrier layers and FGIMD layers 227, 228, or 229.Adjacent metal layers are connected at selected openings by vias 226.Deposited over metal layer 246 are planarized passivation layers 230.

It should be understood that simplified integrated circuit 200 is forillustrative purposes. One of ordinary skill in the art could implementuse of the present invention in relation to fabrication of otherintegrated circuits such as microprocessors, application specificintegrated circuits (ASICs), memory devices, and the like. Further, thepresent invention may be applied to PMOS, NMOS, CMOS, bipolar, orBiCMOS, among other devices.

Exemplary Methods of Forming Graded Dielectric

Referring to FIG. 3, a flowchart is shown illustrating the step of amethod of forming a graded dielectric layer according to embodiments ofthe invention. The method may start by flowing a gas mixture at aninitial flow rate 302 through a gas showerhead to deposit an oxide-richportion of the graded dielectric layer 304 on an underlying layer (e.g.,a low-κ barrier layer). The gas mixture may include a silicon-carboncontaining gas (e.g., OMCTS), an oxygen containing gas (e.g., O₂) and acarrier gas (e.g., He).

The silicon containing gas may be a viscous liquid at room temperatureand the flow rate of its gas vapors measured in milligrams per minute(mgm). When the silicon-carbon containing gas is OMCTS, its initial flowrate may be for example, 500 mgm in a He carrier gas flowing at a rateof about 4800 sccm and O₂ flowing at about 500 sccm during thedeposition of the oxide-rich portion of the graded dielectric layer.

When the oxide-rich portion of the graded dielectric layer is formed,the relative amounts of the gas mixture components are changed to formportions of the dielectric layer with a higher carbon to silicon ratio(i.e., carbon-rich portions of the graded dielectric layer). Thesechanges may include increasing the flow of the carbon-silicon gas to afirst intermediate rate (e.g., increasing the flow rate of OMCTS fromabout 500 mgm to about 1000 mgm) and maintaining that rate for about 0.5seconds or longer (e.g., from about 0.5 seconds to about 2.5 seconds).

Breaking up the increase in flow rate of viscous silicon-carbonprecursors like OMCTS into one or more intermediate steps, reduces thechances of the precursor condensing in the passages of the showerhead,and/or forming aerosol droplets that deposit on the underlying layer. Inthe embodiment illustrated in FIG. 3, the silicon-carbon containing gasis increased to a second intermediate flow rate 308 (e.g., from about1000 mgm to about 1750 mgm) for 0.5 seconds or longer before increasingto a final, fastest flow rate. Other embodiments have the silicon-carbongas flow rate stop for a period of time at additional intermediate flowrates (not shown) before reaching the fastest flow rate.

While the flow rate of the carbon-silicon gas increases in a modulatedfashion, the flow rates for the oxygen containing gas and/or carrier gasmay be continuously decreasing. For example, as an OMCTS flow rateincreases from about 500 mgm to about 1000 mgm, the O₂ flow rate maydrop from about 500 sccm to about 160 sccm and the He gas flow rate maydrop from about 4800 sccm to about 1000 sccm. The flow rates for one ormore of the gases other than the silicon-carbon gas may decreasecontinuously from an initial flow rate to a final flow rate withoutstopping at any intermediate steps. For example, following thedeposition of the oxide-rich layer, the flow rate for these gases maydrop at a constant and continuous rate (without having an intermediaterate plateau) until reaching a final flow rate for the rest of thedeposition.

After the silicon-carbon gas has moved through intermediate flow rates306 and 308, it increases to a final, fastest flow rate 310. Forexample, an OMCTS flow rate may increase from the second intermediaterate (e.g., about 1750 mgm) to a fastest flow rate (e.g., about 2500mgm) where the fastest flow rate may be maintained for 45 seconds orlonger during the deposition of the carbon-rich portion of the gradeddielectric layer 312.

FIG. 4 shows a flowchart illustrating other embodiments for forming agraded dielectric layer according to methods of the invention. In theseembodiments, the flow rate of the silicon-carbon containing gasincreases continuously from an initial rate to a final, fastest ratewithout pausing at an intermediate step. Condensation and/oraerosolization of the silicon-carbon containing gas is avoided, however,by maintaining the flow rate of the carrier gas at a high initial rateuntil after the silicon-carbon containing gas is close to (or reaches)its fastest flow rate.

Similar to the embodiments described above, the method may start byflowing a gas mixture at an initial flow rate 402 through a gasshowerhead to deposit an oxide-rich portion of the graded dielectriclayer 404 on an underlying layer (e.g., a low-κ barrier layer). The gasmixture may include a silicon-carbon containing gas (e.g., OMCTS), anoxygen containing gas (e.g., O₂) and a carrier gas (e.g., He).

After the oxide-rich portion of the layer is formed, the silicon carboncontaining gas is increased in step 406 from the initial flow rate(e.g., about 500 mgm) to a fastest flow rate (e.g., about 2500 mgm)without pausing at an intermediate flow rate. The increase in thesilicon-carbon containing gas flow rate may be, for example, about 1350mgm/sec, such that the flow rate continuously increases from the initialflow rate to the final flow rate in about 1.5 seconds.

Once the silicon-carbon containing gas approaches (e.g., within about500 mgm of fastest rate) or reaches the fastest flow rate, the flow rateof the carrier gas may be reduced to a final flow rate 408 for thedeposition of the carbon-rich portion of the graded dielectric layer410. For example, a carrier gas that includes helium (He) may maintainan initial He flow rate of about 4800 sccms while the OMCTS flow rateincreases from about 500 mgm to about 2500 mgm. When the OMCTS flow rateapproaches (e.g., about 2000 mgm) or reaches (e.g., about 2500 mgm) itsfastest flow rate the He flow rate drops from the initial carrier gasflow rate to a final flow rate (e.g., about 1000 sccm).

In some embodiments, the oxygen containing gas may drop while thesilicon-carbon containing gas is increasing to its fastest flow rate. Inother embodiments, the oxygen containing gas may maintain its initialflow rate with the carrier gas until the silicon-containing gasapproaches or reaches its fastest flow rate. For example, the flow ratefor O₂ may decrease from about 500 sccm to about 160 sccm while the flowrate for OMCTS increases from 500 mgm to 2500 mgm.

FIG. 5 plots the flow rate of a silicon-containing gas over the courseof a dielectric deposition using a conventional method (dashed line) anda method according to an embodiment of the invention (solid line). Theconventional method increases the flow rate of the silicon-carbon gasfrom an initial rate of 500 mgm to a fastest flow rate of 2500 mgm in asshort a time as possible (e.g., about 0.2 seconds). While thesilicon-carbon gas is increasing, the carrier gas (not shown) is rapidlydecreasing, and there is a high probability of forming a lot (e.g.,about 1000 or more) of infilm adders on the underlying layers (e.g., thebarrier layer).

In contrast, the plot of the increase in flow rate for thesilicon-carbon containing gas according to the present invention (solidline) shows the gas going through two intermediate flow rate plateaus,with each plateau lasting about 2 seconds. As noted above, theintermediate steps reduce the chances of the precursor condensing in thepassages of the showerhead, and/or forming aerosol droplets that depositon the underlying layer.

Exemplary Showerhead & Deposition System

FIG. 6 illustrates a sectional view of the structure a showerhead 600that may be used with embodiments of the present invention. A blockerplate 602 having a plurality of through-holes 604 is disposed between aface plate 606 of a shower head 600 and a connecting portion of a gassupply inlet 608. With the blocker plate 602, gas delivered from the gassupply inlet 608 is temporarily stored in a baffle space 610 on theupstream side of the blocker plate 602.

Thus, the irregularity of dynamic pressure of the gas delivered from thegas supply inlet 608 decreases. Consequently, the flow amount of the gasthat flows in the through-holes 604 becomes almost equal. The resultantgas is supplied to a shower pre-chamber 612 on the downstream side ofthe blocker plate 602.

Thus, the irregularity of the pressure in the shower pre-chamber 612decreases. Consequently, the flow amount of the gas sprayed from theface plate 606 through the face plate through-holes 614 becomesconstant. The gas is uniformly supplied to the entire surface of anunderlying substrate (not shown).

To cause the flow amount of the gas that flows in the through-holes 604to be equal, it is effective to decrease the hole diameters of thethrough-holes 604 so as to increase the flow pressure loss of thethrough-holes 604. When the hole diameters of the through-holes 604 aredecreased, the pressure in the baffle space 610 rises and thereby theirregularity of the pressure due to the dynamic pressure of the gasdecreases. In addition, since the difference between the pressure of theupper portion and the pressure of the lower portion (i.e., the pressurein the baffle space 610 and the pressure in the shower pre-chamber 612)of the blocker plate 602 becomes large. Thus, the pressure at eachposition of the blocker plate 602 becomes almost equal. As a result, theflow amount of the gas that flows in the through-holes 604 becomesalmost equal.

As noted above, rapid increases in the flow rate of viscous precursorssuch as OMCTS can result in condensation of the gas vapors inthrough-holes 604 and/or face plate through-holes 614, which in turn cancreate infilm adders on the underlying substrate. In addition, vaporsstarting to condense as they exit the showerhead 600 may act asnucleation sites that form aerosol particles that fall onto thesubstrate, creating another source of infilm adders.

FIG. 7 illustrates an embodiment of a parallel-plate plasma enhancedchemical vapor deposition (PECVD) system 10 that may be used inconjunction with embodiments of the methods and systems of the presentinvention. System 10 includes a vacuum chamber 15 in which one or morelayers may be deposited on a substrate (not shown). System 10 contains agas distribution showerhead 11 for dispersing process gases throughperforated holes in the face plate of showerhead 11 to a substrate(e.g., a 200 mm wafer, 300 mm wafer, etc.) positioned on susceptor 12.Susceptor 12 is thermally responsive and is mounted on supports 13 suchthat the susceptor 12 (and the substrate) can be controllably movedbetween a lower loading/off-loading position and an upper processingposition 14, which is in proximity to showerhead 11. A center board (notshown) includes sensors for providing information on the position of thesubstrate.

When susceptor 12 and substrate are in processing position 14, they aresurrounded by baffle plate 17 having a plurality of spaced holes 23which exhaust into an annular vacuum manifold 24. Deposition and carriergases are supplied through supply lines 18 into a mixing system 19 wherethey are combined and then sent to showerhead 11. Supply lines 18 foreach of the process gases may include (i) safety shut-off valves (notshown) that can be used to automatically or manually shut-off the flowof process gas into the chamber, and (ii) mass flow controllers 20 thatmeasure the flow of gas or liquid through the supply lines. When toxicgases are used in the process, the several safety shut-off valves arepositioned on each gas supply line in conventional configurations.

The rate at which deposition and carrier gases are supplied to gasmixing system 19 is controlled by liquid or gas mass flow controllers 20and/or by valves. During processing, gas supplied to showerhead 11 isvented toward and uniformly distributed radially across the surface ofthe wafer in a laminar flow as indicated by arrows 21. An exhaust systemthen exhausts the gas via ports 23 into the circular vacuum manifold 24and out an exhaust line 31 by a vacuum pump system (not shown). The rateat which gases are released through exhaust line 31 is controlled by athrottle valve 32.

When performing a plasma enhanced process in system 10, a controlledplasma may be formed adjacent to the substrate by RF energy applied toshowerhead 11 from RF power supply 25. Showerhead 11 may also act as anRF electrode, while susceptor 12 is grounded. RF power supply 25 maysupply single or mixed frequency RF power (or other desired variations)to showerhead 11 to enhance the decomposition of reactive speciesintroduced into chamber 15. The mixed frequency RF power is generated bya high frequency RF generator 40 (RF1) and corresponding match circuit42 and a low frequency RF generator 44 (RF2) and corresponding matchcircuit 46. A high frequency filter 48 prevents voltage generated byhigh frequency generator 40 from damaging the low frequency generator.

Heat is distributed by an external lamp module 26. External lamp heatermodule 26 provides a collimated annular pattern of light 27 through aquartz window 28 onto an annular outer peripheral portion of susceptor12. Such heat distribution compensates for the natural heat loss patternof susceptor 12 and provides rapid thermal and uniform susceptor andsubstrate heating for effecting deposition.

The chamber lining, showerhead face plate, supports 13, and other systemhardware may be made out of materials such as aluminum or anodizedaluminum. An example of such an apparatus is described in U.S. Pat. No.5,000,113 entitled “Thermal CVD/PECVD Reactor and Use for ThermalChemical Vapor Deposition of Silicon Dioxide and In situ Multi-stepPlanarized Process,” issued to Wang et al, an assigned to AppliedMaterials, Inc., the assignee of the present invention, the entirecontents of which is herein incorporated by reference.

A motor (not shown) raises and lower susceptor 12 between a processingposition 14 and a lower, substrate-loading position. Motors and opticalsensors are used to move and determine the position of movablemechanical assemblies such as throttle valve 32 and susceptor 12. Theheater, motors, valves and flow controllers 20 connected to supply lines18, gas delivery system, throttle valve 32, RF power supply 25, and lampmagnet drivers are all controlled by a system controller 34 over controllines 36, some of which are shown in FIG. 7.

System controller 34 controls activities of the apparatus. The systemcontroller executes system control software, which is a computer programstored in a computer-readable medium such as a memory 38. Preferably,memory 38 may be a hard disk drive, but memory 38 may also be otherkinds of memory. The computer program includes sets of instructions thatdictate, for example, the timing, mixture of gases, chamber pressure,chamber temperature, RF power levels, susceptor position, and otherparameters of a process. Other computer programs (e.g., one stored onanother memory device such as a floppy disk or other program storagemedia) may also be used to operate processor 34.

The system controller may include a hard disk drive (memory 38), floppydisk drive and card rack, among other elements. The card rack contains asingle board computer (SBC) processor 37, analog and digitalinput/output boards, interface boards and stepper motor controllerboards. Various parts of system 10 may conform to the Versa ModularEuropean (VME) standard that defies board, card cage, and connectordimensions and types. The VME standard also defines the bus structurehaving a 16-bit data bus and 24-bit address bus.

Experimental Examples

Experimental examples have shown that methods of the invention forforming a graded dielectric layer reduce the number of infilm addersdeposited on an underlying barrier layer. Process conditions for acomparative example of forming a dielectric layer using a conventionaldeposition technique are listed in Table 1-A:

TABLE 1-A Process Conditions for Conventional Dielectric LayerDeposition Process Parameter Initiation Deposition Time Period (sec) 1.543 Heater Temp (° C.) 350 350 Pressure (torr) 5 5 Showerhead to Wafer450 450 Spacing (mils) High Freq. RF Power (watts) 500 500 Low Freq. RFPower (watts) 150 150 OMCTS Flow Rate (mgm) 500 2500 O₂ Flow Rate (sccm)500 160 He Flow Rate (sccm) 4800 1000

Bulk dielectric layers were formed on 300 mm wafers in a plasma CVDreactor using the conventional process parameters described above. Theshowerhead used to disperse the process gases included a 496 holeblocker plate that partitioned a gas supply inlet from the face plate.The face plate used was a standard REV 3 face plate. The depositionresults are described below in Table 1-B.

TABLE 1-B Deposition Conditions for Conventional Dielectric LayerDeposition Deposition Condition Comparative Wafer #1 Comparative Wafer#2 Deposition Rate (Å/min) 7077 7029 Range 157 181.35 Uniformity (1 s)0.83% 1.10% RI 1.4557 1.4542 Infilm adders (<0.16 μm) 14,900 32,300

As Table 1-B shows, for both comparative wafers measured, the infirmadders exceeding 0.16 μm in size where greater than 10,000. The majorityof the adders were concentrated at or near the centers of the wafers(e.g., within 100 mm of the wafer center) and almost no adders werecounted near the wafers periphery.

Experimental runs were then conducted for forming a graded dielectriclayer with a 1-step intermediate OMCTS flow rate according to anembodiment of the present invention. Table 2-A lists some of the processconditions used during the deposition:

TABLE 2-A Process Conditions for Graded Dielectric Deposition: ProcessParameter Initiation Intermediate Deposition Time Period (sec) 1.5 1 42Heater Temp (° C.) 350 350 350 Pressure (torr) 5 5 5 Showerhead to Wafer450 450 450 Spacing (mils) High Freq. RF Power (watts) 500 500 500 LowFreq. RF Power (watts) 150 500 150 OMCTS Flow Rate (mgm) 500 1500 2500O₂ Flow Rate (sccm) 500 160 160 He Flow Rate (sccm) 4800 1000 1000

Similar to the comparative example above, bulk dielectric layers wereformed on 300 mm wafers in a plasma CVD reactor using the processparameters described in Table 2-A. The same showerhead was also used.Deposition results are described in Table 2-B.

TABLE 2-B Deposition Conditions for Graded Dielectric Layer DepositionDeposition Condition Wafer #1 Wafer #2 Deposition Rate (Å/min) 6932 6887Range 186 204 Uniformity (1 s) 1.03% 1.22% RI 1.4559 1.4546 Infilmadders (<0.16 μm) 8 4

As Table 2-B shows, the wafers having graded dielectric layers formedaccording to methods of the present invention had barely detectablenumbers of infilm adders exceeding 0.16 μm in size (i.e., less than 10adders). Similarly low numbers of infilm adders (9 and 16 adders) werecounted for a method that included three intermediate flow rate stepsbetween the initial and final flow rates for the OMCTS component.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Additionally, a number of well known processes and elementshave not been described in order to avoid unnecessarily obscuring thepresent invention. Accordingly, the above description should not betaken as limiting the scope of the invention.

Also, the words “comprise,” “comprising,” “include,” “including,” and“includes” when used in this specification and in the following claimsare intended to specify the presence of stated features, integers,components, or steps, but they do not preclude the presence or additionof one or more other features, integers, components, steps or groups.

1. A system for forming a graded dielectric layer on an underlyinglayer, the system comprising: a showerhead comprising a blocking plateand a faceplate, wherein the showerhead is coupled to a gas supply inletthrough which a process gas comprising a silicon-carbon containing gas,an oxygen containing gas and a carrier gas is introduced into theshowerhead; a liquid flow meter to control a flow rate of thesilicon-carbon containing gas to the showerhead; a mass flow controllerto control the flow rate of the carrier gas; and a controllercomprising: a memory storing information indicative of instructions thatwhen performed result in operations comprising: flowing thesilicon-carbon containing gas at an initial flow rate during theformation of an oxygen rich portion of the graded dielectric layer, thenincreasing the silicon-carbon containing gas flow rate from the initialflow rate to an intermediate flow rate, and maintaining the intermediateflow rate for about 0.5 seconds or longer, and then further increasingthe silicon-carbon containing gas flow rate from the intermediate flowrate to a fastest flow rate to form a carbon rich portion of the gradeddielectric layer, wherein the carrier gas has a carrier gas flow ratethat stays constant until after the silicon-carbon containing gasreaches the fastest flow rate.
 2. The system of claim 1, wherein thesilicon-carbon containing gas is a liquid at room temperature.
 3. Thesystem of claim 1, wherein the silicon-carbon containing gas comprisesoctamethylcyclotetrasiloxane (OMCTS).
 4. The system of claim 1, whereinthe oxygen containing gas comprises oxygen (O₂).
 5. The system of claim1, wherein the carrier gas comprises helium (He).
 6. The system of claim1, wherein the operations further comprise increasing the flow of thesilicon-carbon containing gas from the initial flow rate to the firstintermediate flow rate at about 600 milligrams/sec or less.
 7. Thesystem of claim 1, wherein the operations further comprise flowing thesilicon-carbon containing gas at a second intermediate flow rate forabout 0.5 second or longer, wherein the second intermediate flow rate ishigher than the first intermediate flow rate.
 8. The system of claim 1,wherein the operations further comprise decreasing the flow of theoxygen containing gas when the silicon-carbon containing gas goes fromthe initial flow rate to the first intermediate flow rate.
 9. The systemof claim 1, wherein the operations further comprise decreasing thecarrier gas flow rate by about 50% or more after the silicon-carboncontaining gas reaches the fastest flow rate.
 10. The system of claim 1,wherein the initial flow rate of the silicon-containing gas is about 500mgm and the fastest flow rate is about 3000 mgm.
 11. The system of claim1, wherein the operations further comprise decreasing the carrier gasflow rate from about 5000 sccm to about 1000 sccm after thesilicon-carbon containing gas reaches the fastest flow rate.
 12. Thesystem of claim 1, wherein the system comprises a plasma generationsystem configured to form a plasma from the process gas exiting theshowerhead.
 13. The system of claim 12, wherein the system comprises aplasma enhanced chemical vapor deposition system.